Months After Disappearing, La Niña Returns

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A persistent La Niñathat dominated climate patterns late last
year and early this year was blamed for everything from a
record-breaking tornado season to spring flooding, but seemed as
though it had petered out by mid-spring. Now it's back.

After dying down over the summer, La Niña has re-emerged in the
tropical Pacific Ocean and should gradually strengthen and
continue into winter, according to forecasters at the U.S.
National Oceanic and Atmospheric Administration's (NOAA) Climate
Prediction Center.

The strong 2010-11 La Niña contributed to record winter snowfall,
spring flooding and drought across the United States, as well as
other
extreme weather events throughout the world, such as heavy
rain in Australia and an extremely dry equatorial eastern Africa.

La Niña is a naturally occurring climate phenomenon located over
the tropical Pacific Ocean and results from interactions between
the ocean surface and the atmosphere. During La Niña,
cooler-than-average Pacific Ocean temperatures influence global
weather patterns. La Niña typically occurs every three to five
years, and back-to-back episodes occur about 50 percent of the
time. Current conditions reflect a re-development of the June
2010 to May 2011 La Niña episode.

La Niña winters often see drier-than-normal conditions across the
southern tier of the United States and wetter-than-normal
conditions in the Pacific Northwest and Ohio Valley.

"This means drought is likely to continue in the drought-stricken
states of Texas, Oklahoma and New Mexico," said Mike Halpert of
the Climate Prediction Center. "La Niña also often brings colder
winters to the Pacific Northwest and the northern Plains, and
warmer temperatures to the southern states."

According to climate and weather experts,
2011's record-breaking number of billion-dollar weather
disasters was caused by a combination of factors, including
La Niña, local atmospheric patterns and potentially climate
change — though the importance of climate in any individual
weather scenario is still nearly impossible to put an exact
number on. One NASA scientist even called " La
Nada" — the disappearance of La Niña — the real wild weather
culprit.

La Niña does have a strong link to hurricane season, and
forecasters factored the potential return of La Niña into NOAA's
updated 2011 Atlantic hurricane season outlook, issued in August,
which called for an active hurricane season. With the development
of tropical storm Nate this week, the number of tropical cyclones
entered the predicted range for this year of 14 to 19 named
storms.